Remotely operable actuators for flow control valves are well known in the art of fluid distribution and control. Such valve-actuator combinations find their greatest utility in harsh environments, automatic flow distribution systems, and other circumstances wherein a human operator is not typically available to manually change the flow control status of the valve.
For control valves operable between a first flow control state and a second flow control state, such as a gate valve being operable between fully open and fully closed positions, it is common to use a controllable supply of pressurized hydraulic fluid, such as liquid or gas, as a control influence to cause an actuator to depress or withdraw the valve stem of a rising stem gate valve in order to selectably effect the opening and closing of the subject valve. One particular configuration utilizing pressurized hydraulic fluid involves a hydraulic piston or the like oriented coaxially with the rising stem and arranged so as to drive the stem either inwardly or outwardly responsive to the supply of sufficient hydraulic control pressure. Such configurations typically also provide a return spring for driving the valve stem in the opposite direction upon release or failure of the pressurized control fluid supply.
Such valves, termed "fail-safe" due to the feature which returns the valve flow control state to a known condition in the absence of adequate control pressure, find particular utility in oil or gas production applications wherein it is very desirable to know the flow system configuration with certainty during periods of control system damage or equipment failure. For example, a fail-safe valve of the type discussed hereinabove may be placed in the main flow line of a remote gas or oil production or distribution facility and configured so as to typically remain hydraulically driven into the open flow control state by sufficient pressure hydraulic control fluid or gas. During normal system operation, the main gas or oil line valve would remain open, possibly with other downstream valveactuator combinations being used to direct and distribute the flow. Should a failure occur in the control system and control hydraulic pressure be lost, the main gas and/or oil line control valve would be urged into a closed position by the return spring, thus shutting off all flow to the downstream distribution system possibly preventing an overpressure, spill, equipment damage or other undesirable consequences.
Prior art valve-actuator combinations of this general type are disclosed in UK Patent Application No. GB 2 115 111, dated Sept. 1, 1983, by Akkerman and Vazquez, and U.S. Pat. No. 3,789,875, dated Feb. 5, 1974 by McGee. Each of these cited documents shows a valve actuator which is operable to either raise or lower the valve stem of a rising-stem gate valve responsive to a control influence and which further includes a spring means for driving the valve stem in the opposite direction upon release or other removal of the control influence.
As will be appreciated by those skilled in the art, it is desirable, for those applications wherein the ability of such valves to automatically move into a "failure" flow control state upon loss of control system pressure is important, that the return spring rapidly move the valve stem into the return position following loss of control pressure. For hydraulically actuated valves utilizing a liquid control fluid, and particularly for such valves located at a great distance from the source of the control fluid, it will be appreciated that the fluid resistance resulting from the necessity of driving the hydraulic control fluid out of the actuator piston and back through the control fluid supply line can be quite large and can significantly increase the time required for the actuator to move the valve into the desired flow control state. It will also be appreciated that this resistance has the greatest magnitude at the instant of failure when the control fluid is relatively static within the piston and fluid supply line. Additional inertial resistance and mechanical drag are also present in the valve and actuator mechanism. Once these inertial resistances have been overcome and the control fluid has begun to flow from the now moving valve actuator, far less force is necessary to continue the movement of the valve stem into the "failure" position.
Prior art actuators use helical coil springs which must be sized so as to provide adequate force when fully deflected for overcoming the inertial resistance of the control fluid. Once the valve stem is in motion under the influence of this type of spring, however, the constant spring rate of a coil spring results in unnecessary driving force being supplied to the valve stem. For valve actuators utilizing very strong coil springs for overcoming a high initial resistance, the actuator may be quite large and unwieldy.
It is occasionally necessary to access the bonnet retaining means of flow control valves for service, repair, or other purposes. For prior art valve actuators as discussed hereinabove, such access typically requires an almost complete disassembly of the valve-actuator combination. Such disassembly can require lengthy system shutdowns and a prolonged commitment of skilled personnel in order to ensure that such disassembly and subsequent reassembly have been correctly performed.
It may also be necessary and useful in such actuators to include means for manually altering the flow control state of the valve during periods when the control influence has failed or otherwise been removed such as for service or during installation. One critical feature of such local manual actuation systems is that they must not impair the operation of the valve and actuator combination upon restoration of the control fluid pressure. UK Patent Application GB No. 2 115 111, noted above, discloses a prior art actuator having a manual feature disengageable upon repressurization of the control fluid supply line.
What is needed is a fail-safe actuator operable both remotely and locally which is additionally able to overcome the initial inertial resistance of the control fluid in the supply line for returning the valve to a second flow control state upon failure or other removal of the remote control influence. The local manual actuation means must be operable only during periods of control influence failure and must additionally become automatically disengaged upon control influence restoration. Finally, the design of such an actuator should be both simple and compact for maximum industry acceptance and utilization.